It really depends on what type of atmosphere you are looking for in a restaurant.
I've done both for dinner, and enjoyed them both immensely. I would give the edge to Jiko, because that is the one both DH and I ha've been dying to return for dinner!
When I dined at Jiko, DH and I split everything...appetizer, salad, entree, and dessert. Everything was cooked correctly and to our specifications. Our server was *EXCELLENT* and we rewarded him for it....very knowledgable about the food and wine as well. He assisted us in picking out a South African wine called Pinotage made from a SA varietal of the same name. AWESOME!! It was the overall best dining experience on our May/June 2005 trip.
I'm also not a big buffet fan at all. I tend to give them a chance when in WDW or Las Vegas more so than I do anywhere else. Boma, from what I remember, was very good for a buffet. It's been awhile since we have been there...too many places to eat at in WDW!!